The semi-major axis (a) is length 10 and semi-minor axis (b) is length 8 where the standard equation of the ellipse is:
x2/a2+y2/b2=1=x2/102+y2/82=1. A(-10,0) is on the x-axis at the left extremity of the ellipse a distance 'a' from the centre.
If f is the focal distance, then AF1=-f-(-10)=10-f (where F1 is the left focus) and AF2=10+f. So the sum is 10-f+10+f=20 (answer d), because the foci are at F1(-f,0) and F2(f,0).